Discrete Multitone Transmission (DMT) systems are generally known with varieties of Asymmetric Digital Subscriber Line (ADSL) and Very high bit-rate Digital Subscriber Line VDSL, which implement DMT. Technology related to such systems is described in US Patent Publication No. 2001/0004383 (Nordstrom), entitled “DSL transmission system with far-end crosstalk compensation,” and US Patent Publication No. 2001/0006510 (Nordstrom), entitled “DSL transmission system with far-end crosstalk cancellation,” which are hereby incorporated by reference. Such systems may suffer from DownStream (DS) Far-End crosstalk (FEXT) noises effecting DSL modems.
A typical deployment of a DMT system (FIG. 1) may be a Point to Multi-Point configuration where multiple modems are collocated at one side, e.g., Central Office (CO), while the Customer Premises Equipment (CPE) are located at different customer locations and each CPE modem is connected to one of the CO modems. The transmission from the CO modems to the associated CPE modems is known as the Downstream (DS) transmission, and the capacity of the DS transmission is limited in many cases by the interference caused from leakage of the signals transmitted by the different CO modems into each of the CPE modems. This leakage signal is known as Far End Crosstalk (FEXT), thus the received signals at each of the CPE modems contain a mixture of desired signals and FEXT noise signals.
One (simplified) model for the DMT system described above is given by:y=Hx+n  (1)
where x is a vector of symbols, whose coordinates are the transmission symbols along all the modems in the system (in the frequency domain),
y is a vector of received symbols along all CPE modems (in the frequency domain),
H is a channel matrix of complex scalars, where the diagonal elements represent the direct channel frequency response of the desired pair, and the off-diagonal elements represent the FEXT response between different pairs of channels, and
n is the residual noise of the model.
Mitigation of the FEXT noise may be achieved by means of precoding. Several precoding methods have been presented by R. Cendrillon, G. Ginis, M. Moonen, J. Verlinden, T. Bostoen, “Improved Linear Crosstalk Precompensation for DSL,” in IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), May 2004, which is hereby incorporated by reference.
For example, one of the precoding methods comprises multiplying the transmission symbols x by a Cancellation Precoding Matrix F defined as F=H−1diag(H), prior to transmission, where diag(H) is the matrix whose diagonal elements are the diagonal elements of H, and its off diagonal elements are 0. The received symbols y could then be expressed as:y=HH−1diag(H)x+n=diag(H)x+n,  (2)
which is free of FEXT noise.
A necessary condition for implementing precoding solutions is knowledge of the channel matrix H. Once H is known, a Cancellation Precoding Matrix may be computed and precoding methods may be implemented. Common DMT equipment is capable of estimating the diagonal elements of H, but not of estimating the off-diagonal elements of H. Therefore, these methods do not offer a suitable solution for solving the entire channel matrix H.
Reference is made to FIG. 1, which depicts a DSL system 100 including a central office 110 having n transmitting Central Office (CO) modems, and n remotely located Customer Premises Equipment (CPE) modems 120.
Reference is now made to FIG. 2, a block flow diagram describing the transmitting data flow of a Point to Multi-Point DMT system at the CO 200. Multiple streams of data bits are encoded into multiple frequency domain symbols by the Mapper elements 210 of each modem. Typically, each modem processes one stream of data bits into multiple frequency bins, each frequency bin has its associated Mapper 210, which encodes data bits into a frequency domain symbol at the associated frequency bin. Furthermore, typically, the frequency domain symbols are chosen to be one of a predefined set of discrete constellation points, each constellation being defined by the number of points in the constellation and the minimal distance between any two points in the constellation. The frequency domain symbols at each modem independently are transformed into time domain symbols at the Inverse Fast Fourier Transform (IFFT) elements 220 of each modem. A Cyclic Prefix (CP) is added to each time domain symbol at the CP elements 230 of each modem by concatenating a certain predefined number of samples from the end of each time domain symbol to the beginning of each time domain symbol. The time domain symbols are further processed at the Analog Front End (AFE) elements 240 of each modem and transmitted to the various lines.
Reference is now made to FIG. 3, a block flow diagram describing a typical receiving data flow at the various ADSL CPE modems (collectively 300). Typically, each receiver works independently and without access to data in the other modems. Each receiver may convert the analog received signals into digital time domain samples at the AFE elements 310 of each receiver. A time domain filter at the TEQ elements 320 of each modern then preferably processes the time domain symbols. The outcomes of the TEQ elements are further processed by the “Remove CP” elements 330 of each modem, which partition the continuous samples of data into symbols of predefined length and remove a certain predefined number of samples from each symbol. The symbols of time domain samples are then processed by the Fast Fourier Transform (FFT) elements 340 of each modem, which transform the time domain symbols of each modem to frequency domain symbols. The vector of symbols for a specific frequency bin i over all modems is denoted by y. Note that for simplicity we omit the index i, but actually we get a multiple of vector y-s, one for each frequency bin. It will be noted that each CPE typically has access to only one coordinate of the vector y. Frequency Equalizer (FEQ) 350, which is typically a 1 tap complex linear adaptive filter, processes each of the frequency domain symbols to modified frequency domain symbols. The Slicer 360 then processes each of the modified frequency domain symbols into tentative decoded symbols, which are sent for further processing by Error Correction decoders and/or additional elements.
It will be noted that typically, the Frequency Equalizer (FEQ) 350 continuously adapts the values of the 1 tap complex filter using the difference between the modified frequency domain symbols and the tentative decoded symbols as an error estimate and using standard methods such as LMS adaptive filtering. As long as the difference between the modified frequency domain symbols at the receivers and the multiple frequency domain symbols at the Mapper output at the CO transmitters is smaller than half the minimal distance between Constellation points, the Slicer operates correctly. In order to ensure correct operation of the Slicer, the size of the Constellation points and initial values for the TEQ and FEQ parameters are typically set during a training period of the system, which is amply described in the prior art. Typically, the size of the Constellation points and initial values for the TEQ and FEQ parameters are set such that the relationship between each of the transmitted frequency domain symbols at the Mapper output of FIG. 2, denoted x, and each of the corresponding received frequency domain symbols at the FFT output of FIG. 3, denoted y, can be expressed as y=diag(H)x+n, and each coordinate of diag(H)−1n is smaller than half the minimal distance between Constellation points for the corresponding modem. Note that this model differs from model (1) presented above, as its first term diag(H)x does not include FEXT noise, thus the residual noise of this model will include the FEXT noise and will typically be greater than the residual noise in model (1). Further note that preferably such a model is generated for each of the frequency bins, each of which has its corresponding parameters including the channel matrix H, FEQ parameters, and constellation size. The TEQ parameters may typically be common to all the frequency bins, since the TEQ as presented in FIG. 3 operates on the time domain symbols prior to conversion to the frequency domain. However, there exist models in which the TEQ parameters may be set separately for each frequency bin.
A Point to Multi-Point DMT system as described above in FIGS. 2 and 3, where each transmitter modem processes one input stream and generates one output stream is known in the literature as a Single Input Single Output (SISO) system, or to be more precise it is an aggregate of SISO systems.